Purification of aqueous caustic solutions



Patented Jan. 3, 1939 UNITED STATES PURIFICATION OF" AQUEOUS CAUSTIC SOLUTIONS William C. Eichelberger, Syracuse, N.=Y., assignor to The Solvay Process Company, New York N. Y a corporation oi New York No Drawing. Application August 15, 1936; Serial No. 96,248

11 Claims. (or. 204-9) This invention relates to the purification of caustic and is particularly concerned with the removal of chlorate and bromate impurities from caustic such as that produced electrolytically.

In the manufacture of sodium or potassium' hydroxide by electrolysis of the corresponding brine, a small amount of the chloride and, if

present, the bromide appears to be oxidized under the conditions of electrhlysis .so that the caustic product, sodium or'potass'ium hydroxide, contains small quantities, on the order of 0.5% and 0.2% respectively halates, as impurities therein. The presence of these compounds is objectionable if the caustic product is to be applied to certain uses.

It is an object of the present invention to reduce the halate content of caustic products to an unobjectionable amount or to efiect its substantially complete elimination.

In accordance with the present invention it has been found. that the halate impurities in caustic solutions, even though present in exceedingly small amounts, on the order of afraction L or a percent, may be reduced to the corresponding halide by suitable electrolytic treatment of the caustic. During the electrolysis a small amount or a copper salt is continuously or substantially continuously added to the solution being electrolyzed. (01.10.;2fi20) has been found to be highly satisfactory.

The electrolysis apparently forms alkali-metal chloride the cupric chloride and precipitates copper upon the cathode. The onlymaterial introduced into the solution and which is not removed by the electrolysis, therefore, is'the alkali-metal chioi'ide. Since alkali-metal chloride is already present in electrolyticcaustic, the chloride added may be removed therewith by any of the conventional methods for its removal. The rate of addition of the cupric chloride should be regulated to conform to the current passing through the solution. For dilute',caustic a rate of about 2 grams CuCl2-2H2O (representtill) ing V gram copper) per hour per ampere is:

For caustic a rate between most suitable. about /4 and /2 gram oi the cupric salt (representing A to 3 gram of copper) per hour per ampere of current preferably is used. The .addition of the copper salt to the solution should be substantially continuous as a substantial increase or decrease in the rate of addition diminishes the effectiveness of the purification. For purification of sodium hydroxide of around 50% "concentration a rate of cupric chloride addition corof the corresponding 'a substantial period thereafter.

Cupric chloride dihydrate 100 and 110 C.

responding to gram CHCIZZHZO per hour per' ampere has been found satisfactory. In the case of potassium hydroxide solutions the same rate has been found to be very efiective untilthe proportion of halate present has been reduced 5' to about 0.08% calculated as K0103. Below this concentration the rate of addition of cupric chloride should be diminished to equal about gram Cuc12'2H2O per hour per ampere. During the electrolysis of concentrated caustic solutionsv 10 with the continuous addition of cupric chloride, the copper, as noted above, is plated out onto the cathode. The deposit is a spongy formof copper, which, however, does not adhere to the electrode and may be readily washed therefrom w. and settled from the caustic solution. .Since the copper salt is added to the caustic solution until the desired removal of halate has been obtained, the caustic solution would contain a small quantity of copper if the elecso trolysis were immediately discontinued. In order to eliminate this smallproportion of copper, it

is advantageous to continue the electrolysis for In this manner the copper content may be reduced to less than 25 0.041111% and accordingly. for practical purposes the caustic may be regarded as free oi copper. In a series of actual tests on caustic soda liquor, the copper content was found to be not more than 0.0000591, in any case.

The efficiency of purification varies markedly with changes in temperature and it has been found desirable to carry out the process at an elevated temperature. thus, inthe purification of caustic soda solutions it has been found that 35 the rate as well as the completeness of chlorate "removal is increased by increasing the purification temperature from around. 6t (3. to between (With a caustic soda solution of about 8% NaOii, the rate oi chlorate removal is about half as great at o. as at C.

until the chlorate, C103, concentration is reduced to about 0.3 gram per liter; then the relative rate at 60 C. is still lower. With a 50% NaOI-I solution the rate of chlorate removal is 45 about two-thirds as great at 60 (3. as at to C. until the chlorate concentration is reduced to about 2.6 grams per liter; then the rate at 60 C. decreases rapidly so that practically no more chlorate is removed.) Hence it is preferred so to carry out the process with solutions having a causticconcentration around 50% at a temperature .the neighborhood of 10d to 110 0. Higher temperatures may be employedto advantage if desired. In view of the substantial vapor pressure of 50% solutions at higher temperatures, however, it is preferable when such temperatures are employed, to carry out the process under such conditions as to inhibit excessive vaporization. For temperatures in the neighborhood of 100 to 110 C. evaporation may take place but this is' not excessive and the small amount of liquid vaporized may be replaced by the corresponding amount of water to maintain the ,solution concentration constant.

With more concentrated solutions the vapor I out above the boiling point of the solution in which case it may be preferred to maintain the solution at pressures higher than atmospheric pressure to modify or preven ebullition of the solution being treated. 7

While purification of dilute solutions may be effected in accordance with the process of this invention, the efllciency of the process is more or less a function .of the chlorate concentration and hence,for a given ratio of chlorate to caustic a high caustic concentration yields a relatively high emciency. The process is especially effectual for the purification .of solutions containing in the neighborhood of to caustic. As indi-- cated in the preceding paragraph, the purification is satisfactory with more highly concentrated solutions; however, since such solutions are solid at normal temperatures, their handling presents manipulative difliculties, and it is preferred to operate with solutions which are normally liquid.

The electrolysis may be eflected in any suitable apparatus having electrodes'not adversely affected by the solution or causing impurities to be "introduced thereinto.

For this reason it 'is preferred to employ an anode of carbon or simlj larly' inert material which does not go into solu1 tion to any substantial extent during the 'proc+ ess. The cathode material is less important. Nickel, iron, or copper may be employed. The

a use of iron as the cathode has been found to introduce a small amount of iron into the solution and where such impurities are undesirable, and especially-where a product of good color is dey sired, a copper or a copper plated electrode-is to. be preferred.

The efficiency of purification varies to a very substantial degree with variations in the anodic tions good results may be obtained with current current density. In purifying concentratedsoludensities between. about V; and about V ampere' per square inch of anodic surface. In the case of 50% potassium h'ydroxidesolution containing initially on the order of 0.2% potassium halate, when using a /z ampere per ,square inch anodic current density, a halate removal corresponding to 0.22 gram 6103 per liter per ampere was obtained in the first hour and an average rate of 0.08 gram per liter jper ampere hour over the first five hour period; with a V4 ampere per square inch anodic current density a removalequal to 0.32 gram chlorateper liter per ampere was obtained during the first hour and an average rate of 0.16 gram per liter per ampere hour liver the first five hour period; andfor an anodic current density of $4; 'ampere per square inch the rates were 0.30 gram forthe first hour and 0.22 gram over the first five hour period. With dilute sodium hydroxide solutions (-100 grams per liter) the optimum current density was found to 5 be in the neighborhood of 2 amperes per square inch, whereas with 50% sodium hydroxide the optimum was found to be around $4 ampere per square 'inch. A maximum efiiciency for 50% caustic solutions containing an alkali-metal 10 halate concentration in the neighborhood of 0.2% or greater, is obtained using a current density of about $4 ampere per square inch; whereas for reducing alkali-metal halate present only to the extent of a few hundredths percent a ampere 1 per square inch anodic current density gives thehigher efiiciency. It.will be evident, therefore, that the anodic current density may be selected to give a maximum efllciency corresponding to the concentration of caustic and halate present 20' in the solution and may be varied during the electrolysis so as to maintain the efflciency at the optimum. The cathodic current density does .not materially affect the efiiciency of purification and ac- 25 cordingly the cathode area may be subservient to other practical considerations.

During the purification it is desirable to effect agitatiorr of the solution adequate to thoroughly distribute the copper salt added and to prevent 30 polarization of the anode. This agitation may be effected in any suitable manner as by a mechanical stirrer operating within the confines of the electrolytic cell; In some cases the solution may be heated to boiling and the violence of the 35 boiling may be controlled to effect the desired agitatibn. A simple but effective means of pro- A viding adequate agitation and uniform distribution of the copper salt comprises withdrawing a portion of the liquid from the cell as a stream thereof,-iniecting into the stream the requisite quantity of cupric chloride and passing the stream'by means of a propeller or centrifugal pump back into the cell at a point remote from that at which it was withdrawn. The rotary type 5 pump effects a thorough admixture of the salt solution introduced with the liquid circulated and thereby provides a preadmixture of the salt solutlon with the caustic solution, which facilitates uniform distribution in the cell. The solution, I if desired, may be returned to the cell through a-nozzle to give it a relatively high temporary velocity and provide stirring of the contents of the cell. The concentration of the copper salt solution added may be adjusted so that the water supplied thereby to the caustic compensates for the water evaporated from the solution during the electrolytic purification. The amount of water tobe added in-this manner will, of course, depend upon'the particular type of apparatus employed on and the particular conditions under which the process is carried out.

',.The temperature of the solution during electrolysis maybe kept atthe optimum in any suitable manner, thus the caustic solution may be s5 supplied in an initially heated condition and heat losses avoidedby suitable insulation so that'no external heating is necessary. Since the passage f of the currentthigough the solution introduces substantial heat, this is sufficient, where adequate 7o insulation is provided, to maintain the requisite I temperature. Otherwise to compensate for heat losses by'ra .dlation,- conduction, etc. the electrolytic cell may be heatedby the direct application of heat thereto, or, in the case of a circulatory system, the caustic solution may be passed through a heater prior to its return to the cell.

} By minimizing vaporization of water from the solution, heat imized.

The following of the present invention. In the processes 01' Examples. 1 and 2 the electrolytic cell comprises an iron vessel having its inner face copper platedand a cylindrical carbon anode'driven byan' electric motor so as to rotatefand provide agitation of the solution invthe cell; The iron vesselitself was made the cathode.

Example 1.5400'p'arts of an aqueous 49% KOH solution prepared by the electrolysis of a potassium chloride brine and having'a total content of bromate and chlorate corresponding to 3.04 grams per liter or approximately 0.2%

KCIO3 was placed in the vessel and washeated to 107 Cf by direct application of heat to the vessel. The electric current was then started perature was maintained at between 107 and 110 C. and the anodic current density-was maintained substantially constant. At'the end of the 'two hour period the potassium halate content of the hydroxide solution had been reduced to 1.40 grams per liter. The rate of addition of cupric chloride was then reduced to 0.5 gram per hour for each 2 amperes or current going through the cell; otherwise the conditions were maintained'the same; the process thus modified was continued for four hours more. The introduction of cupric chloride solution was then discontinued and the electrolysis was continued for hour longer to eliminate copper from the solution..- This also further reduced the percentage of halatepresent so that the final concentration of halate in the solution corresponded to 0.31

gram per liter or apprommately 0.02% KiJlOs. The removal of KClOs amounted to about 1 gram for each 1.3 ampere hours of current used with a potential drop or about 2.0 volts.-

Example 2-5400 parts by weight of an aqueous electrolytic caustic solution containing about i895,

. ing through the cell. The processwas continued for 22 hours whereby the halate content of the caustic soda solution was "reduced from 0.05 grams NaClOg per liter to 0.71 gram per liter, approximately 017%. The removal of NaClOa amounted to about 1, gram. for each 2 ampere,

with, a P. D. of about 2 hours of current used to 2 volts. I

Example 3.-About 3960 partsby weight of a 10% NaOI-l. solution containing 0.78 gram per liter of halate (measured as C103)v were subjected to electrolysis in an. apparatus of the type loss from this source may be minexamples illustrate the process above described, (except that stirring was accomplished by an auxiliary stirrer instead of by rotating the anode) at boiling temperature (105- 107 C.) for a period of 7.hours using an anodic current density 01' about 2 amperes with an anode area of v2 square-inches. A solution of cucn-zrno, whose co so that the water ther' f compensated for evaporationloses, was added atthe rate of 8 grams CuCh'2H2O per hour, i. e. about 2 grams per ampere of current. At the end of this period 99% of the total chlorate content had been reduced.

I claim: g 1. The method of reducing halate present in aqueous caustic solution containing halate, which comprises subjecting the solution to electrolysis using an anodic current density between about A; and about 4 amperes per square inch and gradually adding a soluble copper salt to the soluntration was adjusted tion during the electroylsis at a rate between about ,5 and about gram copper per hour per ampere of current passing through the solution.

2. The method, of reducing halate present in aqueous caustic solution which comprises subjecting the solution to electrolysis using an 'anodic current density between about $4; and about 2 amperes per square inch and gradually adding a copper salt to the solution at a rate between about ,5 and about *71 gram copper per hour per ampere of current passing through the solution.

3. The method of reducing'the halate content I of a concentrated aqueous caustic solution, whichcomprises subjecting the solution to electrolysis using an anodic current density between about and about ampere per square inch and gradually adding a soluble copper salt to the caustic solution at a rate between about ,6 and about $5 gram copper per hour'per ampere of current passing throu h the solution.

4.. The method of reducing the halate content or a concentrated aqueous caustic solution, which comprises subjecting the solution while hot to electrolysis using an anodic current density between about A-and about ampere per square inch and gradually adding. a soluble, copper salt to the caustic solution at a rate between about ,4 and about gram copper per hour per ampere of current passing through the solution.

5. The method of reducing the halate content of a concentrated aqueous caustic solution, which comprises subjecting the solution at a tempera-' ture between about and about C. to electrolysis using an anodic current density between about %3 and about /2 ampere per square inch and gradually addinga 'solublecopper salt to the caustic solution at a rate between about ti and about 3i -gram copper per hour per ampere of current passing through the solution.

0. The method of reducing the halate content of a concentrated aqueous caustic solution, which comprises subjecting the solution to electrolysis using an anodic current density between about A; and about V ampere per square inch and gradu ally adding cupric chloride to the caustic solution of aqueous potassium hydroxide'solution containing about 50% KOi-i and about 0.2% ofpotassium halate, which comprises subjecting the solution at a temperature between about 100 and about solution.

110 C. to electrolysis using an anodic current density between about A; and about A; ampere per square inch and gradually adding cupric chloride to the solution at a rate corresponding to about gram copper per hour per ampere of current passing through the solution.

8. The method of reducing the halate content .of aqueous potassium hydroxide solution containing about 50% KOH and about 0.2% of potassium halate, which comprises subjecting the solution at a temperature between about 100 and about 110 C. to electrolysis using an anodic current density between about A; and about /4 ampere per square inch and gradually adding cupric chloride to the solution at a rate corresponding to about gram copper per hour per ampere of current passing through the solution, continuing said electrolysis until the potassium halate content of the solution has been reduced to about .08% and thereupon diminishing the rate of addition of cupric chloride tov about ,5 gram copper per hour per ampereof current passing through the 9. The method of reducing the halate content of an aqueous sodium hydroxide solution containing about NaOH and about 0.5% sodium halate, which comprises subjecting the solution at a temperature between about 100 and about 110 C. to electrolysis using an anodic current density between about and about ampere per square inch and gradually adding cupric chloride to the solution at a rate of about 35 gram copper per hour per ampere of current passing through the solution.

10. The method of reducing the halate content of a dilute aqueous caustic solution, which comprises subjecting the solution at a temperature between C. and the boiling point of the solution to electrolysis using an anodic current density between about 2 and 4 amperes per square inch and gradually adding cupric chloride to the caustic solution at a rate corresponding to about ti gram copper per hour per ampere of current passing through the solution.

11. The method of reducing the halate content of an aqueous sodium hydroxide solution containing about 10% NaOH, which comprises subjecting the solution at a temperature between C. and the boiling point of the solution to electrolysis using an anodic current density between about 2 and about 4 amperes per square inch and gradually adding cupric chloride solution to the sodium hydroxide solution at a rate corresponding to about 3 gram copper per hour per ampere of current passing through the solution.

' WILLIAM C.'EICI-IELBERGER. 

